Geosynthetic clay liners made with bentonite are used to form impermeable barriers in landfills and man-made bodies of water. The present invention addresses a problem associated with the manufacture of such geosynthetic clay liners. Specifically, the present invention addresses the problem of how to get the bentonite granules to adhere to the primary carrier sheet, the cover sheet as well as to adjacent bentonite granules in order to form a flexible yet cohesive liner.
By way of background, a geosynthetic clay liner (GCL) is comprised of three primary structural components: (1) the primary carrier sheet, also known as the primary backing, primary textile or primary carrier; (2) the cover sheet, also known as the secondary textile, secondary carrier sheet or scrim; and (3) a layer of granular bentonite disposed therebetween.
The primary carrier sheet is normally the lower sheet of material that engages the ground or surface on which the GCL is installed. The cover sheet is normally the upper sheet of material that engages either the landfill material, water or other material that is deposited on top of the GCL once installed. Bentonite is a naturally occurring clay material which tends to expand upon being exposed to water. When a layer of dry bentonite is wetted, the bentonite layer is capable of substantially precluding the migration of liquids through the bentonite layer if it is properly contained. One way to ensure proper containment of the bentonite material in a landfill or man-made body of water is to provide the bentonite in the form of a GCL whereby the primary carrier sheet and cover sheet act to contain the granular bentonite after it is wetted and ensure that an impermeable barrier is created by the swollen layer of bentonite clay.
The present invention addresses problems associated in preventing movement or displacement of the granular bentonite once the GCL is constructed. Specifically, considering that a GCL comprises a layer of granule bentonite disposed between a primary carrier sheet and a cover sheet, it is evident that movement or displacement of the granule bentonite between the primary carrier sheet and the cover sheet will occur unless some means for containing or maintaining the position of the bentonite granules is provided.
At least one manufacturer uses glue or adhesive to connect or attach the bentonite granules to the cover sheet and/or to the primary carrier sheet. If this technique is used, the manufacturer is assured that at least a portion of the bentonite granules will be disposed continuously across the cover sheet and the primary carrier sheet, even though some bentonite granules disposed in the middle of the bentonite layer may move or shift during shipment and handling due to insufficient infiltration of the glue or adhesive into the bentonite layer.
The disadvantages to using glue as a means for holding the components of the GCL together are twofold. First, the use of glue increases the cost of manufacture of GCLs because it is a time-consuming manufacturing process. The glue must be applied to the primary carrier sheet before the bentonite is placed on top of the primary carrier sheet. Additional applications of glue to the bentonite and to the cover sheet are also required. Then, the glue must be dried, a procedure which requires expensive equipment and which has significant energy costs. While the use of glue has the benefit of holding the bentonite granules in place, the use of glue as a sole means for holding the liner together may require repeated applications of glue, which is expensive in terms of manufacturing time, and further involves substantial energy consumption in the drying process, which is expensive in terms of energy costs.
Other manufacturers use needle punching and/or quilting as a means for holding a GCL together. The needle punching process requires the use of at least one non-woven fabric for the cover sheet or the primary carrier sheet. The non-woven fibers are pulled by the needles to interlock with the adjacent fabric sheet. Non-woven fabrics have lower tensile strengths than woven fabrics and have higher lateral transmissivity than woven fabrics and therefore non-woven fabrics are not as preferable for use in GCLs as woven fabrics. Non-woven fabrics are simply not as strong or as durable as woven fabrics and therefore needle punching non-woven fabrics together with a layer of bentonite disposed therebetween is not a preferable method of manufacturing GCLs.
Quilting is another method of holding the bentonite layer in place. In a quilted GCL, the primary carrier sheet and cover sheet are sewn together in an intersecting pattern. The bentonite is held in place in sewn pockets or compartments. Quilting is not preferable because of the excessive amount of sewing required which drives up the cost of the finished product. Further, within each pocket formed by a quilt-like pattern, there may be shifting of the bentonite. This can result in locations of high permeability.
Thus, it would be highly advantageous to the geosynthetic clay liner industry if a process for manufacturing geosynthetic clay liners was provided that (1) avoided the use of glue or adhesives altogether, (2) avoided the use of needle punching, (3) avoided quilting or equivalents thereof. The present invention makes a significant contribution in this respect by providing a manufacturing process that utilizes woven primary carrier and cover sheets or the use of film-type material such as polyethylene or polypropylene for the primary carrier and cover sheets and which relies on the adhesive properties of moistened bentonite itself to hold the bentonite in place. The edges of the primary carrier and cover sheets may be sewn together in a conventional manner; needle punching, quilting or gluing is not required. It should be noted that it may be advantageous to add longitudinally disposed rows of stitching between upper and lower fabrics to improve the shear resistance of the GCL when it is expected that the GCL will be used on a slope. The longitudinal stitching, however, is not intended to provide any significant resistance to localized shifting of the bentonite granules during transport of the GCL from the point of its manufacture to the site at which it is to be installed.